This paper presents a microfluidic device specially designed for DNA tile self-assembly. The DNA tile is one of the most promising building blocks for complex nanostructure, which can be used as a molecular computer or a scaffold for functional molecular machineries. In order to build desired nanostructure, it is necessary to realize errorless self-assembly under thermal fluctuation. We propose a method to directly control environmental parameters of DNA self-assembly such as concentration of each monomer tile and temperature in the reaction chamber by using a microfluidic device. The proposed device is driven by a capillary pump and has an open reaction chamber which enables real-time observation by AFM. Results of preliminary experiments to evaluate performance of the device will be reported.